The present invention relates to a headbox to uniformly distribute a reactive, aqueous suspension of calcium sulfate hemi-hydrate crystals and fibers across the width of a foraminous wire mesh in a water-felting process to form a uniform gypsum/fiber web. More specifically, the headbox of the present invention may be used to distribute hot suspensions of calcium sulfate hemi-hydrate crystals and lignocellulose fibers, such as paper fibers or wood fibers, although the headbox of the present invention also may be used to distribute suspensions of other dense crystals and any fibrous materials.
The process of water felting dilute aqueous dispersions of various fibrous materials is a well-known commercial process for manufacturing many types of paper and board products. In this process, an aqueous dispersion of fiber, binder and other ingredients, as desired or necessary, is flowed onto a moving foraminous support wire, such as that of a Fourdrinier or Oliver mat forming machine, for dewatering. The dispersion may be first dewatered by gravity and then dewatered by vacuum suction means; the wet mat is then pressed to a specified thickness between rolls and the support wire to remove additional water. The pressed mat is then dried in heated convection or forced air drying ovens, and the dried material is cut to the desired dimensions.
U.S. Pat. No. 5,320,677 to Baig describes a composite material made from gypsum and host particles of a reinforcing material, such as lignocellulose fibers, hereinafter sometimes referred to as gypsum/wood fiber board. The composite material is produced by mixing gypsum and host particles of a stronger substance, such as wood fibers, in a dilute aqueous slurry. The slurry is heated in an autoclave, preferably under pressure, to convert the gypsum to calcium sulfate alpha hemihydrate. The hot, converted slurry is discharged through a headbox onto a continuous felting conveyor of the type used in paper making operations, where the slurry is dewatered to remove as much uncombined water as possible before rehydrating the hemihydrate back to gypsum. The resulting material is a homogeneous mass comprising gypsum crystals physically interlocked with the discrete host particles. The resulting mat is then dried in heated convection or forced air drying ovens, and the dried board is cut to the desired dimensions.
In order to produce the best composite product it is essential to spread the hot, reactive gypsum/fiber slurry uniformly across the width of the felting conveyor. The headbox of the present invention functions to spread the aqueous gypsum/fiber slurry over a moving wire screen much like the headboxes used in papermaking. However, the aqueous gypsum/fiber slurry differs from conventional papermaking stock in that the aqueous gypsum/fiber slurry is hot and reactive and has much higher solids than traditional papermaking slurries. The aqueous gypsum/fiber slurry to which the present invention pertains is comprised of two materials having markedly different densities. For example, the gypsum/fiber slurry may contain as much as 30% by weight of solids, of which from 80-97.5% by weight is high-density gypsum and 2.5-20% by weight is low-density paper fibers. Because of the disparate density of the components, the aqueous gypsum/fiber slurry must be constantly agitated, but the agitation must be in a manner that avoids breaking down the friable acicular calcium sulfate hemihydrate crystals. The particular nature of the aqueous gypsum/fiber slurry imposes special requirements on the headbox design.
In the formation of gypsum/fiber board products using such a water felting process, one measure of board quality is the formation property, that is, the degree of uniformity of distribution of solid components or fibers in the finished board. This formation property, while dependent upon many factors inherent in individual water felting process machines, is determined to a large extent by the flow pattern of gypsum/fiber stock from the sluice of the headbox onto the Fourdrinier wire or the web formation surface, and by the flocculated or unflocculated state of the fibers in the gypsum/wood fiber stock flow. Thus, if the gypsum/fiber stock flow onto the formation surface is uneven across the width of the surface, is uneven throughout the course of the machine run, or if the gypsum/fibers deposited onto the formation surface are in a non-uniformly flocculated state, the finished board will show stripes, ripples, density spots, strength variations and the like, characteristic of nonuniform distribution of fiber components. It is obvious, of course, that in the manufacture of quality board the formation property must be stringently controlled to insure a completely uniform distribution of gypsum/fibers onto the formation surface both by presenting an even stock flow and unflocculated fibers in the stock flow. Accordingly, control of the evenness of the stock flow and the flocculation of the fibers is very important inasmuch as even slight variations in gypsum/fiber stock flow or slight variations in the flocculation of the gypsum/fibers are sufficient to introduce serious non-uniformity to the finished board.
It is the general object of the present invention to provide a headbox for a Fourdrinier machine that produces a uniform distribution of the gypsum/fiber stock across the width of the Fourdrinier wire.
A more specific object of the Invention is to provide a headbox for a Fourdrinier machine having an improved structure for controlling the factors contributing to the formation property of the gypsum/fiber board produced thereby, particularly where low slurry flow rates and/or high solids contents are required.
Another object of the Invention is to provide improved turbulence generating distribution rolls by means of which the formation property of a gypsum/fiber board may be better controlled.
Another object of the invention is to provide improved distribution rolls for the headbox of a Fourdrinier machine by means of which sufficient controlled turbulence is introduced into the gypsum/wood fiber stock flowing therein to maintain the gypsum/wood fiber stock in a controlled state of flocculation.
Another object of the invention is to provide a distribution roll for the headbox of a Fourdrinier machine having an improved grid structure, preferably a diamond pattern, by means of which fibers in the gypsum/fiber stock are maintained in a controlled state of flocculation and the velocity profile of the gypsum/fiber stock flow is smoothed.
An additional object of the invention is to provide improved turbulence generating distribution rolls having sufficient openness to effectively smooth the velocity profile of the gypsum/wood fiber stock flowing in the headbox and of sufficient rigidity so as to be substantially unflexible when supported at its ends for rotation about its longitudinal axis.
A more specific object of the invention is to provide improved turbulence generating distribution rolls for which the surface solidity is less than 80% and which is sufficiently rigid to prevent flexing when the structure is supported at its ends for rotation about its longitudinal axis.
Another specific object of the invention is to provide improved turbulence generating distribution rolls having a solidity of between 65% and 80% of the area of its cylindrical surface, that is, an openness of between 20% and 35%.
The present invention is a headbox for distributing a hot aqueous dispersion of calcium sulfate hemi-hydrate crystals and non-gypsum fiber uniformly across the width of the forming wire of a Fourdrinier machine or the like. The headbox of the present invention is described herein in the context of distributing a hot suspension of gypsum crystals and lignocellulose fibers, such as paper fibers or wood fibers, as described in U.S. Pat. No. 5,320,677 to Baig. However, the headbox of the present invention may also be used to distribute suspensions of other dense crystals and any fibrous material.
The headbox generally comprises a housing and two horizontal, counter-rotating, perforated distribution rolls that extend substantially across the width of the Fourdrinier forming wire. The housing includes a first curved section that is shaped to match the curvature of the cylindrical surface of the first distribution roll. The housing includes a second curved section that is shaped to match the curvature of the cylindrical surface of the second distribution roll. The two curved sections extend across the width of the headbox.
The distribution rolls are positioned in the housing wherein the cylindrical surface of the distribution rolls is closely spaced to the curved section of the housing. The rotation of the roll imparts energy to the fluid in the boundary layer of the slurry in contact with the roll. The rotating roll thus causes a pumping and mixing action in the slurry located in the gap between the rotating distribution roll and the static curved section of the housing and thus provides the desired controlled agitation of the gypsum/fiber stock without high shear. The level of agitation is a function of the roll speed and the spacing between the moving surface of the roll and the adjacent static curved section.
As the space between the roll and the curved section increases, the viscous drag decreases. As a result, the spacing between the rotating roll and the static curved section is important. As this spacing increases at the same roll rpm, the pumping and mixing action of the slurry in the spacing decreases. To maintain the same pumping and mixing action as the spacing between the roll and curved section increases, the roll rpm can be increased. In practice, the spacing between the roll and curved section is minimized so that the greatest range of roll speed can be utilized. The spacing is practically determined by the minimum spacing that does not encounter plugging of large clumps of slurry solids entering the headbox. Preferably, a weir formed by the intersection of the two curved sections separates the distribution rolls.
An overhead manifold feeds the hot aqueous dispersion of gypsum crystals and fiber, at multiple points across the width of the headbox, to a chute adjacent to the space between the housing curved section and the first distribution roll. The first distribution roll rotates toward (countercurrent) the incoming aqueous dispersion, while the second distribution roll rotates with (concurrent) the aqueous dispersion flowing over the weir.
The distribution rolls are both preferably perforated. The first roll, which is preferably larger, e.g. 10 inches (254 mm), has a plurality of round holes, preferably set in a diamond pattern. Preferably, the holes in the first roll are about 1 inch (25.4 mm) in diameter. From about 25 to about 35% of the cylindrical surface of the first roll may be perforated, i.e. open space, and preferably about 30% of the first roll is open space.
The second roll, which is smaller, e.g. 7 inches (179 mm), has a plurality of round holes set in a regular pattern. Preferably, the holes in the second roll are about 1xc2xd inch (38.1 mm) in diameter. From about 20 to about 30% of the cylindrical surface of the second roll may be perforated, and preferably about 25% of the second roll is open space. The speed of the roll rotation may be independently varied to control flow patterns, but a peripheral speed of about 120 to 180 feet per minute (36.6 to 54.9 meters per minute) is preferred.
A second embodiment of the headbox of the present invention is also disclosed. In this embodiment, a first rotating horizontal distribution roll is also positioned in the housing, and a second rotating horizontal distribution roll is positioned in the housing, but the first distribution roll is more or less vertically disposed beneath said second distribution roll. In this embodiment the curved sections are closely spaced to the most of the surface of the corresponding rolls e.g. 270xc2x0 or 300xc2x0. The curved sections of the housing extend around the rolls and intersect along two parallel lines to form a throat between the rolls that extends across the width of said rolls.
It is to be understood that the foregoing general description, and the following detailed description, are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a partial disclosure of the present invention, illustrate two specific embodiments of the headbox of the present invention.